The drawings of air separation units (ASU) producing the oxygen intended for an oxy-coal power plant generally comprise two vaporisers (or even three) located between the medium-pressure column (MP column) and the low-pressure column (LP column). The installation of these two vaporisers makes it possible to reduce the pressure of the MP column to a value of about 3 bar absolute, which makes it possible to minimise the energy consumption of the ASU.
The purity of the oxygen produced by this type of power plant is typically between 95 and 97 mol % O2. The vaporisation of the oxygen is performed in a dedicated vaporiser. The vaporisation frigories of the liquid oxygen are used to condense the gaseous air. A method of this type is known from U.S. Pat. No. 4,936,099 and EP-A-0547946.
Moreover, one can attempt to take advantage of the installation of such an ASU to produce pure liquid nitrogen and pure oxygen (purity of about 99.6%), stored and then intended for liquid trade via lorries.
The production of liquid nitrogen does not give rise to any major difficulty, as it is sufficient to add plates at the top of the MP column in order to achieve the desired purity, without impacting the rest of the ASU process, except for the cost of the liquefaction energy.
However, the production of pure oxygen (>99.6%) induces a more substantial impact on the method; indeed, the purity of the liquid produced is clearly superior to that of the gaseous oxygen supplied to the oxycombustion power plant. It is therefore necessary to install a small additional column, that recovers a fraction of the liquid flow collected in the LP column (in the vessel or on an intermediate plate), distilling same, which makes it possible to recover, at the bottom of this small additional column, the pure oxygen intended for trade by lorries. The gaseous return from the pure LOX column is then carried out at the same level as the tapping of the liquid in the LP column.
Nevertheless, the pressure of the MP column is so low that it is not possible to use one of the gaseous flows entering or exiting the MP column or the LP column to be condensed in the vessel vaporiser of the column of pure additional LOX (the condensation temperature thereof is too low).